1. Field of the Invention:
The present invention relates to a moving magnet type meter wherein a movable magnet is caused by perform an angular motion by virtue of a magnetic field generated in coils when energized, thereby operating the pointer.
2. Description of the Prior Art:
Moving magnet type meters are disclosed, for example, in U.S. Pat. No. 4,760,333 and Japanese Utility Model Publication No. 19801/1988. FIG. 5 shows an example of such conventional moving magnet type meter, in which a bobbin body 3 is constituted by upper and lower bobbins 1, 2 formed of a synthetic resin; a movable magnet 4 is disposed within the bobbin body 3; the upper end of a pointer shaft 5 inserted through the movable magnet 4 is projected from the upper bobbin 1, with a pointer 6 being fixed to the thus-projecting upper end of the pointer shaft; a plurality of leg portions 7 formed at the outer edge of the bobbin 3 projectingly in diagonal positions, the leg portions 7 being each provided with a terminal 8 connected electrically to the exterior such as a power source; leader lines of coils 9 wound crosswise round the bobbin body 3 are connected to the terminals 8; and the movable magnet 4 is caused to perform an angular motion by virtue of a combined magnetic field generated in response to electric currents flowing through the coils 9, thereby causing the pointer 6 to make indication.
In the above moving magnet type meter, the pointer shaft 5 and the movable magnet 4 disposed within the bobbin body 3 formed of a synthetic resin are supported in predetermined positions through bearings so as to be rotatable smoothly.
As such bearing structure, a lower bearing 10 which supports the lower end portion of the pointer shaft 5 in the radial direction is disposed nearly centrally of the lower bobbin 2, while an upper bearing 11 which supports the upper end side of the pointer shaft 5 in the radial and thrust directions is provided in the upper bobbin 1.
In this connection, a reduced-diameter portion 12 is formed at the upper end portion of the pointer shaft 5 and a thrust bearing portion 14 is formed by a stepped part 13 defined by the reduced-diameter portion 12 and the inner wall portion of the upper bobbin 1 through a slight gap, so a vertical clearance, namely, a thrust gap, for the pointer shaft 5 is set between the bearing portion of the lower bobbin 2 and the thrust bearing portion 14 of the upper bobbin 2, and outer peripheral portions 1' and 2' of the bobbin body 3 are formed flat. Therefore, due to a change in temperature during or after the mounting of the coils 9 or due to an external pressure load induced by the mounting of the coils, the outer peripheral portions 1' and 2' of the bobbin body 3 will be deflected inwards, resulting in that the movable components (movable magnet 4 and pointer shaft 5) disposed within the bobbin body 3 are displaced in thrust clearance and hence extinguished by the deflection of the bobbin body 3, so the bobbin body 3 side and the movable component side come into contact with each other, thus causing the movable components to become inoperative.
Moreover, the wound state of the coils may be changed by such deformation of the bobbin body 3, causing a change in the magnetic torque exerted on the movable magnet 4 and resultant deviation in the indication characteristic.
Further, in the above conventional structure, it is necessary to form the stepped part 13 at the upper end portion of the pointer shaft 5, for example by cutting, so much labor is required for the fabrication of such part; besides, since the pointer 6 is fixed to the reduced-diameter portion 12 by such means as press fitting, there is a tendency to deteriorated strength.